We plan to use explants of fetal rat liver in organ culture to examine how fetal rat liver develops the ability to generate tyrosine aminotransferase in response to corticosteroids, other hormones, and cyclic AMP. We will investigate the development of p-hydrophenylpyruvate oxidase and other components of the "tyrosine oxidizing system" of perinatal liver in order to establish if change in tyrosine aminotransferase activity is an indication of changes occurring in the complex of enzymes responsible for tyrosine catabolism. We will examine placental and pituitary factors to determine whether they affect induction of tyrosine metabolizing enzymes by nuclear binding macromolecules or receptors during preincubation with the development of inducing capability of steroids.